Excavation machines of various descriptions find application in the installation, removal, and repair of below and above ground utilities and structures. Typical below ground utilities include water mains, sewers, conduit for electrical and communications lines; electrical and communications lines installed without conduit, subway transit tunnels, water tunnels and the like.
Below ground installation of utilities such as electrical and communication lines removes the utility lines from the visual appearance of the landscape. The location of underground utilities is generally established in advance by design engineers and provided to persons installing the utilities in the form of drawings. Location includes not only the X-Y-axes location of the utility with respect to the surface of the earth, but also includes location on the Z-axis, e.g., the distance beneath the surface of the earth or possibly referenced to sea level. In practice, the actual location of underground utilities may deviate from the location described in preconstruction drawings because of interference below the surface of the earth resulting from rocks, or rock formation, trees, building foundations or previously installed utilities unknown to the design engineers. In anticipation of the installation of additional below ground utilities and structures in the vicinity of a first structure, and in anticipation of possible repair or replacement of a first underground utility in a vicinity, and to prevent subsequent excavations from encountering unmarked sub-surface utility structures or sub-surface obstructions, engineers make a record of the location of the utility, as installed and possibly other sub-surface obstructions. Such locations are recorded on drawings known as “as-built drawings”.
Initial or crude as-built drawings edited in the field may be red-line notations on a copy of the design drawings, or in a preferable circumstance, the location of the as-built utility may be established by hand measurements and surveying instruments. Notes of such measurements, if they are preserved and located, may be subsequently transferred to as-built drawings now more often in electronic form by means of computer aided design tools such as the program AutoCad™ or similar computer aided design tools.
The instant invention finds utility with excavation machines including tracked excavators, wheel-based excavators, and tractor-based backhoes. It is known to determine the location of an excavator, or other machine for adjusting and moving surface and below surface earth by means of global positioning devices (GPS). The GPS device determines the location of its antenna. If the antenna is located on the machine, then the machine location may be determined with relation to satellite triangulation. The location of a feature on a job site requires location of the GPS antenna at that location. While the location of sub-surface “as-built” features on a job site may be found by locating GPS antennas at such features, such a task has limited advantages over hand measurements and surveying instruments. Notes of measurements and transfer of the as-built measurements to drawings remains a requirement. Typically, the as-built drawings will be a condition precedent to final payment to a builder or contractor by a utility company or municipality. Furthermore, GPS signals may be obstructed within an a below ground level excavation, or by neighboring building structures or terrain.
“Offsets” provide a useful addition to GPS location information. An offset is the distance, direction, orientation, and depth (or height) of a feature determined with respect to the location of the GPS antenna. When the offset is combined with GPS location, the location of the feature can be identified in three coordinates. Identification of two points on a target feature discloses the orientation of the feature as well as the location of the feature.